Wiper problem

I have a strange wiper problem where they vary the travel distance constantly. Generally at low / delay speeds they will only travel about 3/4 of the "full" amount before returning. At high speed however they do travel the full distance. It seems that possibbly the timing device or switch is malfunctioning.

Does anyone know how to troubleshoot this, and what part to look at, in particular what controls the timing interval ?

Thanks

Anyone ?

Still hoping to get some help with this...